Many of the measurments of Cardiovascular Nuclear Medicine have uncertainties and limitations that are directly related to the physics of radiation detection. In vivo measurements, however, are complicated to analyze because of the many factors that interact in a synergistic manner. We propose to investigate the physical characteristics of the detection process in the laboratory using computer simulations, phantoms, a dynamic (pumping) heart model, and animal studies. Collimation, optimum radioisotope energy, pulse height analysis, background correction procedures, countrate to ventricular volume relationships, temporal resolution, and detector field-of-view are a few of the instumentation parameters that will be examined. Inputs from this study will be applied directly to improve the design of radioisotope detection systems for measuring information what left ventricular volume/time curves; these curves are important because they contain information that can be processed to yield a number of performance characteristics for assessing cardiac function. As a final step, the improved detection systems will be employed to record and evaluate left ventricular volume/time curves on patients routinely undergoing 99mTc-HSA blood pool imaging.